Hydrogenated nitrile rubber, also known by the abbreviated term HNBR, is prepared via hydrogenation of nitrile rubber, also known by the abbreviated term NBR.
Nitrile rubbers, also known by the abbreviated term NBR, are rubbers which are copolymers composed of at least one unsaturated nitrile and of at least one conjugated diene and possibly of other comonomers.
Hydrogenated nitrile rubber is a specialty rubber which has very good heat resistance, excellent resistance to ozone and chemicals, and also exceptional oil resistance.
The abovementioned physical and chemical properties of HNBR are associated with very good mechanical properties, in particular with high abrasion resistance. For this reason, HNBR is now widely used in a very wide variety of application sectors. HNBR is used, for example, for gaskets, hoses and damping elements in the automobile sector, and also for stators, borehole seals and valve seals in the oil extraction sector, and also for numerous electrical-industry components, mechanical-engineering components and shipbuilding components.
Marketed HNBR grades have a Mooney viscosity (ML 1+4 @ 100° C.) in the range from 55 to 105, corresponding to a weight-average molecular weight Mw (determined by: gel permeation chromatography (GPC) against polystyrene equivalents) in the range from about 200 000 to 500 000. The polydispersity indices D found here (D=Mw/Mn, where Mw is the weight-average molecular weight and Mn is the number-average molecular weight) indicate the breadth of molecular weight distribution and are greater than 3.0. The residue double bond content is moreover in the range from 1 to 18% (determined via IR spectroscopy).
The relatively high Mooney viscosity places severe restrictions on the processability of HNBR. Many applications would ideally use an HNBR grade whose molecular weight is lower and whose Mooney viscosity is therefore lower. This would give a decisive improvement in processability.
In the past, numerous attempts have been made to shorten the chain length of HNBR via degradation. One possibility is mechanical degradation via what is known as mastication, e.g. on a roll mill. Another possibility is chemical degradation, e.g. via reaction with strong acids. However, a disadvantage of this chemical degradation is that functional groups such as carboxylic acid groups and ester groups are incorporated into the molecule, and there is moreover a substantial change in the microstructure of the polymer. All of these changes have attendant disadvantages for the material's applications.
Using the methods currently predominant in standard production processes it is impossible or very difficult to prepare an HNBR with a Mooney viscosity (ML 1+4 @ 100° C.) smaller than 55 and which therefore has improved processability. The familiar preparation process for HNBR is hydrogenation of NBR. This hydrogenation normally increases the Mooney viscosity of the polymer by a factor of 2 or more, the function, inter alia, of the NBR grade used and of the degree of hydrogenation. This means that the viscosity range of marketed HNBR is limited via the lower limit of the Mooney viscosity of the starting material NBR, which nowadays is at values somewhat below 30 MU.
WO-A-02/100941 and WO-A-02/100905 represent the closest prior art. WO-A-02/100905 describes a process which encompasses the degradation of nitrile rubber starting polymers via olefin metathesis and subsequent hydrogenation. In this process, a nitrile rubber is reacted in a first step in the presence of a co-olefin and of a specific complex catalyst based on osmium, on ruthenium, on molybdenum or on tungsten, and in a second step is hydrogenated. According to WO-A-02/100941, hydrogenated nitrile rubbers with a weight-average molecular weight (Mw) in the range from 30 000 to 250 000 and with a Mooney viscosity (ML 1+4 @ 100° C.) in the range from 3 to 50 and with a polydispersity index D smaller than 2.5 are obtainable by way of this process. The resultant HNBR degradation products therefore feature a broad range of possible molecular weights, and also a molecular weight distribution which is relatively narrow in relation to the starting polymer. However, a disadvantage of this metathesis reaction is that it is necessary to use a catalyst which requires complicated preparation and is expensive. Another disadvantage is the fact that the breadth of molecular weight distribution or the polydispersity index has a lower limit by virtue of the nature of the chemical degradation of the molecular weight. In the olefin metathesis, a random chain degradation takes place and leads to a polydispersity of at least 2.0. In “The Polymeric Materials Encyclopedia”, CRC Press, Inc. 1996, V. V. Korshak, “Metathesis polymerization, Cycloolefins”, page 12 says, for example, that a polydispersity index of from 2.01 to 2.23 is achieved via metathesis degradation of unsaturated carbon polymer chains. Values smaller than 2.01 must therefore be values distorted via the measurement errors of the test method.
Starting from the prior art, the object of the present invention consisted in providing a process which can prepare a hydrogenated nitrile rubber which has narrower molecular weight distribution or respectively a smaller polydispersity index than the hydrogenated nitrile rubbers of WO-A-02/100941 previously disclosed and at the same time also has low values for the weight-average molecular weight.